In the process of high-speed circuit board design, electromagnetic compatibility design is an important point and also a difficult point. This paper discusses how to reduce the electromagnetic interference caused by conduction coupling and radiation coupling and improve electromagnetic compatibility from the aspects of layer number design and layer layout.
Many reliability and stability problems of electronic products are caused by poor electromagnetic compatibility design. Common problems include signal distortion, excessive signal noise, unstable signal during work, system crashes easily, system is susceptible to environmental interference, and poor anti-interference ability. Electromagnetic compatibility design is a rather complex technology, from design to electromagnetics and other aspects of knowledge. This article discusses some empirical skills from layer design and layer layout, and provides some references for electronic engineers.
The layers of the PCB board mainly include power supply layer, ground layer and signal layer, and the number of layers is the sum of the number of each layer. In the design process, the first step is to coordinate and classify all sources and grounds, as well as various signals, and deploy and design based on the classification. In general, different power supplies should be divided into different layers, and different grounds should also have corresponding ground planes. Various special signals, such as clock high and high frequency signals, need to be designed separately, and ground planes need to be added to shield special signals to improve electromagnetic compatibility. When cost is also one of the factors to be considered, a balance point must be found between the electromagnetic compatibility and cost of the system in the design process.
The first thing to consider in the design of the power layer is the type and quantity of the power supply. If there is only one power supply, a single power layer can be considered. In the case of high power requirements, there can also be multiple power layers to supply power to devices of different layers. If there are multiple power supplies, consider designing multiple power supply layers, or divide different power supplies on the same power supply layer. The premise of division is that there is no crossover between power supplies. If there is crossover, multiple power supply layers must be designed.
The design of the number of signal layers should take into account the characteristics of all signals. Layering and shielding of special signals are limited considerations. Under normal circumstances, it is first designed with design software, and then modified according to specific details. Both the signal density and the integrity of special signals must be considered in the design of the number of layers. For special information, be sure to design the ground plane layer as a shield if necessary.
Under normal circumstances, if the cost is not purely considered, it is not recommended to design single-sided or double-sided panels. Although the processing of single-sided and double-sided panels is simple and cost-effective, but in the case of relatively high signal density and complex signal structure, such as high-speed digital circuits or analog-to-digital hybrid circuits, since the single-sided panel does not have a dedicated reference ground layer, the loop The area increases and the radiation increases. Due to the lack of effective shielding, the anti-interference ability of the system is also reduced.
The layout design of the PCB board layer, after the signals and layers are determined, the layout of each layer also needs to be scientifically designed. The layout design of the middle layer of PCB board design follows the following principles:
1. Align the power planes adjacent to the corresponding ground planes. The purpose of this design is to form a coupling capacitor and work together with the decoupling capacitor on the PCB to reduce the impedance of the power plane and obtain a wider filtering effect.
2. The selection of the reference layer is very important. In theory, the power layer and the ground plane can be used as the reference layer, but the ground plane layer can generally be grounded, so the shielding effect is much better than the power layer, so in general, the ground plane is preferred as the reference layer. Reference plane.
3. Key signals on two adjacent layers cannot cross partitions. Otherwise, a large signal loop will be formed, resulting in strong radiation and coupling.
4. To maintain the integrity of the ground plane, it is not allowed to route on the ground plane. If the signal line density is too high, you can consider routing on the edge of the power layer.
5. Design the ground layer under key signals such as high-speed signals, test signals, and high-frequency signals, so that the path of the signal loop is the shortest and the radiation is minimal.
6. In the process of high-speed circuit design, how to deal with the radiation of the power supply and the interference to the whole system must be considered. Generally, the area of the power layer plane should be smaller than that of the ground plane, so that the ground plane can shield the power supply. It is generally required that the power plane is indented by 2 times the dielectric thickness than the ground plane. If the indentation of the power plane is to be reduced, the thickness of the dielectric should be kept as small as possible.
General principles to be followed in the layout design of multilayer printed boards:
1. The power layer plane should be close to the ground plane and designed below the ground plane.
2. The wiring layer should be designed to be adjacent to the entire metal plane.
3. The digital signal and the analog signal should have an isolation design. First of all, avoid the digital signal and the analog signal on the same layer. Digital signal area isolation. The same is true for analog and digital power supplies. Especially the digital power supply has very large radiation, so it must be isolated and shielded.
4. The printed lines in the middle layer form a planar waveguide, and the microstrip line is formed in the surface layer. The transmission characteristics of the two are different.
5. Clock circuits and high-frequency circuits are the main sources of interference and radiation, so they must be arranged separately and away from sensitive circuits.
6. The stray current and high-frequency radiation current contained in different layers are different, and they cannot be treated equally when wiring.
The electromagnetic compatibility of the PCB board can be greatly improved through the design of the number of layers and the layout of the layers. The design of the number of layers mainly considers the power layer and ground layer, high-frequency signals, special signals, and sensitive signals. Layer layout mainly considers various couplings, ground and power line layouts, clock and high-speed signal layouts, analog signals and digital information layouts.
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